1. Field of the Invention
The present invention relates to a signal processor, a method of signal processing and a computer program, and in particular to those preferably applicable to processing of measured value of physical quantities based on analog-to-digital signal conversion.
2. Description of the Related Art
There are conventionally known measurement devices capable of measuring physical quantities such as noise, vibration and so forth, comparing measured results of the physical quantities with reference values, and displaying the compared results on display devices. This sort of measurement devices can readily inform the user whether the measured physical quantities exceed the reference value or not.
FIG. 6 shows a conventional configuration of a noise level meter as one example of such measurement devices. As shown in FIG. 6, the noise level meter is configured as having a microphone 501, a pre-amplifier 502, an A/D converter 503, a DSP (digital signal processor) 504, a CPU (central processing unit) 505, an operation section 506, and a display device 507.
A sound signal detected and converted to a voltage signal by the microphone 501 is amplified by the pre-amplifier 502, and thereby an analog sound level signal is generated. The analog sound level signal is converted to a digital sound level signal by the A/D converter 503, and is output to the DSP 504. A root mean squared value calculation circuit 504a provided to the DSP 504 derives a root mean squared value (linear value) of sound level from the digital sound level signal output from the A/D converter 503, and outputs the result to the CPU 505.
A sound level conversion circuit 505a provided to the CPU 505 converts the root mean squared value of sound level output from the root mean squared value calculation circuit 504a into a dB (decibel) value (log value), and outputs the result to the display device 507, and also to a comparison circuit 505b provided to the CPU 505. The display device 507 displays the dB value of sound level output from the sound level conversion circuit 505a. 
On the other hand, the comparison circuit 505b compares the dB value of sound level output from the sound level conversion circuit 505a, with a dB value of sound level set, in response to operation made on the operation section 506, by a sound level setting circuit 505c provided to the CPU 505. More specifically, the comparison circuit 505b compares the measured value of sound level based on sound detected by the microphone 501, with a reference value of sound level set by the user using the operation section 506. The comparison circuit 505b then outputs the result of comparison to the display device 507. The display device 507 displays the result of output from the comparison circuit 505b, together with the above-described dB value of sound level.
As has been described in the above, the measurement devices such as noise level meter uses both of the DSP 504 and the CPU 505 for a variety of operations. Speed of processing is, however, generally larger in the DSP 504 than in the CPU 505, so that it is necessary for the DSP 504 to output signals to the CPU 505 at predetermined intervals (10 ms to 100 ms, for example).
In the exemplary noise level meter shown in FIG. 6, the sound level determined by the root mean squared value calculation circuit 504a is output to the sound level conversion circuit 505a at regular intervals. Therefore, the root mean squared value of sound level based on the sound detected by the microphone 501 cannot be input to the sound level conversion circuit 505a in a real-time manner, and instead a certain duration of time is necessary before the dB value of sound level is obtained by the sound level conversion circuit 505a. For this reason, the comparison circuit 505b is made wait before starting its operation, until the dB value of sound level is input to the sound level conversion circuit 505a. This configuration may result in a large time lag between the point of time when the sound was picked up by the microphone 501, and the point of time when the comparison took place in the comparison circuit 505b or when the display took place in the display device 507, and may interfere with correct measurement.
As has been described in the above, the related art has been suffering from a problem in that it would be impossible to compare the measured value of a physical quantity and the reference value of the physical quantity in a real-time manner.